Automatic amplitude control for oscillator circuits



June 1, 1943. F. s. MABRY AUTOMATIC AMPLITUDE CONTROL FOR OSCILLATORCIRCUITS Filed Dec. 30, 1941 INVENTOR Torrey 5 M957 ATTORNE PatentedJune 1, 1943 UNITED STATES AT ENT OFFICE AUTOMATIC AMPLITUDE CONTROL FOROSCILLATOR CIRCUITS- Application December 30, 1941, Serial No. 424,898

4 Claims.

My invention relates to electric discharge tube oscillators controlledby electromechanical vibrators and, in particular, relates to theprovision of an automatic arrangement for regulating the amplitude ofthe oscillations produced by the tube.

Oscillators in which an electromechanical vibrator such as apiezo-electric crystal of quartz or other suitable material is connectedbetween the anode and the grid of a tube containing at least threeelectrodes, and in which electrical oscillations of substantially thenatural frequency of vibration of the vibrator are produced in thecircuit of the principal electrodes are well known in the prior art.Such arrangements operate quite satisfactorily where only moderateamounts of power output are required from the tube. However, when sucharrangements are employed to produce large power outputs, for example,by employing relatively high plate supply voltages, considerableirregularities and variances were found in the operation of differentvibrators. In some cases, the vibrators oscillate so violently as to beoverheated; in other cases, relatively feeble oscillations only can beproduced.

One object of my invention is to provide an ancillary arrangement bywhich the amplitude of the oscillations of the electromechanicalvibrator can be controlled and regulated to prevent such over-violent orunder-powered oscillations.

Another object of my invention is to provide apiezo-electricaliy-controlled oscillator with automatic means forregulating the amplitude of oscillation of the piezo-electric' crystalat will.

Other objects of my invention will become apparent upon reading thefollowing description taken in connection with the single figure ofdrawing which is a schematic diagram of a circuit embodying theprinciples of my invention.

Referring in detail to the drawing, an electrical discharge tube Ihaving an anode 2, a cathode 3 and at least one control electrode 4, andwhich may be for many purposes of the high vacuum type, is supplied withcurrent from a source of direct-current 5 through a resistor 6, thiscurrent being preferably derived from a tap point I on a potentiometercomprising two resistor sections 8, 9 which are connected across theterminals of the direct-current source 5. The tube I may, with advantagefor many purposes, be of the Variable Mu type well known in the art. Therelative magnitudes of the resistors 8, 9 are so proportioned as toimpress between the cathode 3 of the tube I and its control electrode Ia suitable negative bias voltage in a manner well known in the radioart. Resistors II, I2 and I3 are serially connected between the negativeterminal of the direct-current source 5 and the control electrode 4 ofthe tube I. Between the anodei and the control electrode 4 are connecteda capacitor I4 in series with an electromechanical vibrator such as apiezo-electric crystal I5 of a type well ,known in the radio art. A loadcircuit may be connected through a capacitor .16 between the anode 2 andthe cathode 3. The circuit arrangement so far described is aconventional one for producing electrical oscillations between the anodeand cathode of the tube I.

In order to provide for automatic regulation of the amplitude of thevibrations of the vibrator I5 in accordance with my invention, I providea second electrical discharge tube- II which may in many cases be of thehigh vacuum type and which has an anode I8, a cathode I9 and at leastone control electrode 2|. Theanode I8 is connected through a resistor 22to the positive terminalof the direct-current source 5, and the cathodeI9 is connected to the common junction of the resistors I2 and I3previously mentioned; as positioned in the grid circuit of the tube I. Acapacitor 23 is preferably connected in shunt between the anode and thecathode of thegtube IT. The control electrode2l is connected tothecommon junction of the resistors II and I2 previously mentioned inthe control electrode circuit of the tube I. A capacitor 24' isconnected in shunt to the resistor I2.

The mode of operation of the above-described circuit to regulate theintensity of the vibration of the vibrator I5 is as follows: a It willbe noted that any current which flows from the generator 5 through thetube I'I, resistor 22 and resistor I3 will produce a voltagedrop tendingto make the control-electrode l of the tube I more positive. It willalso be noted that the voltagedrop through the resistor I3 will thus beopposite in direction to the voltage impressed upon the controlelectrode fl by the current flow in the resistor 3. -It will likewisebe, noted that the resistor I2 is traversed by the electric currentresulting: from rectification of the electric oscillations on grid 4.Thus the grid voltage applied to tube I7 is proportional to the strengthof the oscillations. Now suppose that some change occurs whichtemporarily increases the amplitude of oscillation. The result of thiswill be an increased rectified current traversing resistors 8, I3, I2,II and control grid 4 and cathode 3 thereby increasing the negativepotential at grid 4. The negative potential on grid 2| will have beenincreased which in turn will decrease the current through resistors 22and I3. The decrease of current in resistor I3 will decrease the buckingpotential thereof and allow the negative potential on control grid 4 togo still farther negative thereby tending to bring the amplitude ofoscillation back to the normal state. The action of capacitors 23 and 24is to by-pass high frequency currents which might be present due tostray fields and etc. and make tube ll react only to the D. C.potentials impressed.

It will readily be seen that any decrease in the strength of thevibrations in vibrator I5 from a given value decreases the negative biasapplied by the resistor I2 to the control electrode 2| of tube l'l,thereby increasing the positive bias applied by the resistor I3 to thecontrol electrode 4 and thus increasing the strength of the oscillationsproduced by the tube I. In short, the reaction of the circuits ofancillary tube I1 is such as to tend to neutralize any variance of thestrength of the vibrations of crystal I5 from a given normal value.

The initial adjustments of the above-described system to determine theabove-mentioned normal value may be made as follows: With the tube I!and the vibrator I5 removed from circuit, the magnitude of theresistance 8 should initially be adjusted to reduce the plate current ofthe tube I to a very low value. The resistors I3 and 22 should then beadjusted to produce a bias which would cause tube I to operate as aclass A amplifier when not producing oscillations. Then connecting thecrystal I5 in position, the resistors II and I2 should be adjusted sothat the plate current of tube I! has a relatively small value when thecrystal I5 vibrates with the desired or normal amplitude.

The ancillary tube I! and its circuits are applicable not only toregulate the intensity of vibrations of the tube I when it is operatedwith a crystal control, but may likewise be used to control theamplitude of its oscillations when the vibrator I5 is removed and thetube connected to oscillate in an ordinary feedback connection or withan oscillation-excited grid circuit, both the latter connections beingwell known in the radio art.

While I have described a particular embodiment of my invention, itsprinciples are of broader application in ways that will be readilyevident to those skilled in the art.

I claim as my invention:

1. In combination, an electrical discharge tube having at least twoprincipal electrodes and a control electrode, means connected to supplypower to said principal electrodes, an electromechanical vibratorconnected in circuit between said control electrode and one of saidother principal electrodes, a resistance means connected between saidcontrol electrode and the other of said principal electrodes, a secondelectrical discharge tube having at least two principal electrodes and acontrol electrode connected to cause current flowing between itsprincipal electrodes to traverse a portion of said resistance means,

means for connecting another portion of said resistance means to impressa bias voltage between the control electrode and the principal electrodeof said second electrical discharge tube, and means for supplyingcurrent to the principal electrodes of said second electrical dischargetube.

2. In combination with an electrical discharge tube having at least twoprincipal electrodes and a control electrode, an electromechanicalvibrator and a capacitor connected in shunt between one of saidprincipal electrodes and said control electrode, resistance means havinga plurality of sections connected between said control electrodes andthe other of said principal electrodes, a direct current sourceconnected to cause current flow between said principal electrodes, asecond electrical discharge tube connected to said current source andhaving at least an anode, a cathode and a control electrode, means forconnecting said anode to the positive terminal of said direct-currentsource, means for connecting said cathode to a first tap point on saidresistance means, and means for connecting the last-mentioned controlelectrode to a second tap point on said resistance means which is nearerto the firstmentioned control electrode than is said first tap point.

3. In combination, a first electrical discharge tube having an anode, acathode and a control electrode, means for connecting said anode to thepositive terminal of a direct-current source, means for connecting saidcontrol electrode through a resistance-means to the negative terminal ofsaid direct-current source, and means for connecting said cathode to apoint intermediate in potential between the terminals of. saiddirectcurrent source, a capacitor and a piezo-electric crystal connectedbetween said anode and said control electrode, a second electricaldischarge tube having an anode, a cathode and at least one controlelectrode, means ,for connecting said cathode to a first tap point onsaid resistancemeans, means for connecting the last-mentioned controlelectrode to a second tap point on said resistance-means which is closerto the first-mentioned control electrode than is said first tap point, acapacitor shunting said tap points, and means for connecting thelast-mentioned anode to derive current from the positive terminal ofsaid direct-current source.

4. In combination with an electrical discharge tube having a principalelectrode circuit and a control electrode circuit, means for supplyingexciting current through an electromechanical vibrator to said controlelectrode circuit, a second electrical discharge tube having principalelectrodes and a control electrode circuit, means for impressing avoltage varying with said exciting current on the last-mentioned controlelectrode circuit, and means for impressing a voltage varying with thecurrent between the last-mentioned principal electrodes on thefirst-mentioned control electrode circuit.

FORREST SUMMERS MABRY.

